题目
You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
给定两个非空的链表，表示两个非负整数。 数字以相反的顺序存储，每个节点包含一个数字。 添加两个数字并将其作为链表返回。 您可以假设两个数字不包含任何前导零，除了数字0本身。
也就是从头加到尾，需要注意的是如何处理不同长度的数字，以及进位和最高位的判断。这里对于不同长度的数字，我们通过将较短的数字补0来保证每一位都能相加。递归写法的思路比较直接，即判断该轮递归中两个ListNode是否为null。
全部为null时，返回进位值有一个为null时，返回不为null的那个ListNode和进位相加的值都不为null时，返回 两个ListNode和进位相加的值
代码
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
return helper(l1,l2,0);
}
public ListNode helper(ListNode l1, ListNode l2, int carry){
if(l1==null && l2==null){
return carry == 0? null : new ListNode(carry);
}
if(l1==null && l2!=null){
l1 = new ListNode(0);
}
if(l2==null && l1!=null){
l2 = new ListNode(0);
}
int sum = l1.val + l2.val + carry;
ListNode curr = new ListNode(sum % 10);
curr.next = helper(l1.next, l2.next, sum / 10);
return curr;
}
}
完整运行程序：
package AddTwoNum;
class ListNode{
int val;
ListNode nextNode;
ListNode(int val){
this.val = val;
this.nextNode = null;
}
}
public class AddTwoNum {
public ListNode addTwoNumbers1(ListNode l1, ListNode l2) {
return helper(l1,l2,0);
}
public ListNode helper(ListNode l1,ListNode l2,int carry){
if(l1 == null && l2 == null){
return carry == 0? null : new ListNode(carry);
}
if(l1 == null && l2 != null){
l1 = new ListNode(0);
}
if(l2 == null && l1 != null){
l2 = new ListNode(0);
}
int sum = l1.val + l2.val+carry;
ListNode curr = new ListNode(sum);
curr.nextNode = helper(l1.nextNode, l2.nextNode, sum/10);
return curr;
}
public static void main(String[] args) {
int []input1 = new int []{2,4,3};
int []input2 = new int []{5,6,4};
ListNode listNode1 = buildListNode(input1);
ListNode listNode2 = buildListNode(input2);
AddTwoNum addTwoNum = new AddTwoNum();
ListNode result = addTwoNum.addTwoNumbers1(listNode1,listNode2);
while(result != null){
System.out.print(result.val);
result = result.nextNode;
if(result !=null){
System.out.print("->");
}
}
}
private static ListNode buildListNode(int[] input){
ListNode first = null,last = null,newNode;
int sum;
if (input.length>0){
for(int i = 0;i<input.length;i++){
newNode = new ListNode(input[i]);
newNode.nextNode = null;
if(first == null){
first = newNode;
last = newNode;
}else {
last.nextNode= newNode;
last = newNode;
}
}
}
return first;
}
}
